30 A PRIMER OF BIOLOGY 



one ? We have already said that one type of 

 energy may be converted into another. Physicists 

 tell us that thermal energy is the only one into 

 which all the others are convertible. For this 

 reason energy is usually measured in terms of heat, 

 and the unit of measurement is known as a " calorie." 

 A calorie is the amount of heat required to warm one 

 kilogram of water from 0C. to 1C. and it is possible 

 to measure the energy of every body possessing it 

 in terms of this unit. The energy of the living 

 organism, as well as the energy of the various food 

 substances absorbed by it, may, therefore, be esti- 

 mated in calories. ; ' The heat value of a substance 

 is the amount of heat that is produced by its complete 

 oxidation, and this amount is the same whether the 

 oxidation be quick or slow, reached by a direct or 

 by a circuitous path. It is, therefore, possible to 

 estimate the amount of heat that must be produced 

 in the body, by estimating the heat-value of the 

 food daily consumed." (Waller). Thus, if the heat- 

 value of 1 gr. of proteid be 5 calories, of 1 gr. of fat 

 9'07 calories, and of 1 gr. of starch 3 g 9 calories, the 

 heat-value of the nitrogenous and non-nitrogenous 

 food iorming the diet of an average man doing 

 average work (p. 28) for twenty-four hours must be, 

 approximately, 3,300 calories. The total energy of 

 the body appears (a) as work, (b) as heat, and it 

 has been found that these bear to each other a 

 ratio of about 1 : 4, so that the measurable heat 

 of the body will amount roughly to about 2,640 

 calories. These values are only approximate, 

 since some organic compounds are excreted from 

 the body in an incompletely oxidised condition 

 still possessing for that reason a certain heat- 

 value. 



